This invention relates to a process control system, and more particularly to a process control system having two-degrees of freedom for proportional control operation.
Proportional and Integration (PI) or Proportional, Integration and Derivative (PID) control devices have been widely utilized in all fields of production, since the inception of process control. Indeed, in the field of instrumentation, PID control devices have become indespensable.
In the past, various kinds of control systems have been proposed. A shift has now taken place from analog systems to digital control and calculating systems. However, the leading role of plant operation control in the application of PID control devices remains unchanged.
The basic equation of such PID control calculation is: ##EQU1## where MV(s) is the operating signal, E(s) is the deviation, Kp is the proportional gain, T.sub.I is the integral time, T.sub.D is the deviation time, s is the Laplace operator, and (1/.eta.) is the derivative gain.
Such PID control calculation is called a PID control calculation system with one degree of freedom.
This constitutes a so-called system with one degree of freedom, in which only one set of PID control constants can be set.
It is of course impossible to optimize simultaneously both the external disturbance suppression characteristic and the target value tracking characteristic.
In general, a control system employing PI or PID control operations must satisfy the functions of both an external disturbance suppression characteristic and a target value tracking characteristic.
The former, the external disturbance suppression characteristic, indicates how best to suppress the effect of external disturbances.
The latter, the target value tracking characteristic, indicates how best to track the target value of the process value when this target value is changed.
In an ordinary PI or PID control system, the values of the PI or PID control constants for optimal suppression of the effect of changes in external disturbance and the values of the PI and PID control constants for optimal tracking of changes in target value are very different, so that these two characteristics can not be optimized simultaneously. In fact, they are in an antinomic relationship.
Specifically, if the PID control constants are set so as to optimally suppress the effect of changes in external disturbances, the target value tracking characteristic becomes oscillatory. On the other hand, if the PID control constants are set so as to optimally track changes in target value, the external disturbance suppression characteristic becomes very soft.
Development of a technique for simultaneously optimizing the external disturbance suppression characteristic and target value tracking characteristic of such PID control devices is therefore desirable.
To deal with this, in 1963, Issac M. Horowitz announced the basic concept of a two degrees of freedom PI or PID (herein below abbreviated as 2 DOF PID) algorithm in which two sets of PID control constants could be independently set. 2 DOF PID systems based on this concept have been implemented in Japan also in recent years, and are making a large contribution to improvement of plant operation control.
FIG. 1 is a block diagram of a conventional two degrees of freedom type PID control system wherein P operation and D operation are given two degrees of freedom. In this system, there is provided a target value filter 11 that carries out calculation processing to obtains the proportional gain and derivative time in the two degrees of freedom type system. The system receives a target value SV from a target value generating unit 10 and feeds to a deviation calculation unit 13 a control amount PV from a system 12 to be controlled and filter output SVo obtained from target value filter 11. Deviation calculation unit 13 performs the calculation (SVo-PV) to find the deviation E, which is then fed to a PI control calculation unit 14. This PI control calculation unit 14 receives deviation E and performs a PI control calculation to obtain an operating signal MV, which is then fed to an adding unit 15. This adding unit 15 combines operating signal MV and an external disturbance signal D by adding them, and applies the combined added signal which is obtained to the controlled system 12, thereby performing control such that target value SVo=control amount value PV.
In target value filter 11, coefficient unit 21 multiplies target value SV by proportional gain revision coefficient .alpha. for realizing two degrees of freedom for proportional control operation, to obtain a output (SV.multidot..alpha.), which is then fed to an adding unit 22. A subtraction unit 23 then subtracts the output (SV.multidot..alpha.) of coefficient unit 21 from target value SV. The subtraction output SV(1-.alpha.) which is obtained is then supplied to a subtraction unit 24.
The output (SV.multidot..alpha.) of the coefficient unit 21 is also fed to a coefficient unit 25 which multiplies this output (SV.multidot..alpha.) by derivative time revision coefficient .gamma. for realizing two degrees of freedom of derivative control operation to obtain a multiplied output (SV.multidot..alpha..multidot..gamma.), which is then fed to a subtraction unit 26.
This subtraction unit 26 subtracts control amount PV from multiplied output (SV.multidot..alpha..multidot..gamma.), to obtain a subtracted output (SV.multidot..alpha..multidot..gamma.)-PV, which is fed to a coefficient unit 27, where this subtraction output (SV.multidot..alpha..multidot..gamma.)-PV is multiplied by derivative gain 1/.eta.. The output of this coefficient unit 27 is branched into two outputs. One of these outputs is input directly to a subtraction unit 28. The output is input to the negative input of subtraction unit 28 through a first order delay element 29.
In subtraction unit 28, the output of first order delay element 29 is subtracted from the output of coefficient means 27, and the result is then supplied to subtraction unit 24. This subtraction unit 24 then subtracts the output of subtraction unit 28 from the output of subtraction unit 23. The output obtained is given an appropriate delay by passing it through a first order delay element 30 whose time constant is the integral time T.sub.I. An adding unit 31 then adds the output of this first order delay element 30 and the output of subtraction unit 28. Adding unit 22 then adds this to the output of a coefficient unit 21, to obtain the output SVo of target value filter 11.
The following relationship therefore subsists between the output SVo of target value filter 11 and target value SV and control amount value PV. ##EQU2## As a result, taking the target value SV component in operating signal MV to be MVs, from FIG. 1 and equation (2) above, we have the following equation. ##EQU3## And taking the control amount value PV component in the operating signal MV as MVp, we likewise, obtain from FIG. 1 and equation (2) above the following equation. ##EQU4## Consequently, as is clear from these equations (3) and (4), a simple PID control is achieved of the control amount value PV. But regarding the target value SV, the proportional gain revision coefficient .alpha. and derivative gain revision coefficient .gamma. can be respectively independently set. Thus, so-called two degrees of freedom PID control is achieved.
Although this 2 DOF PID control device has various advantages, the following drawbacks have been pointed out. Specifically, as shown in equation (3) and (4), there are three different ways in which the derivative control operation may be applied to the target value SV and control amount value PV (influenced by external disturbance D), depending on the way in which .gamma. and T.sub.D are set (see Table 2 below). In actual industrial application these combinations may be employed selectively, or may be altered during operation, depending on the characteristics of the process and the requirements of control.
As a result, the values of the coefficient .alpha. used to give the proportional gain revision coefficient for realizing two degrees of freedom differ greatly, depending on which of the above combinations is selected. Consequently, in a plant employing several hundred to several thousand PID control loops, it is necessary to change the setting of the value of coefficient .alpha. every time. This takes a considerable amount of time. It is also possible that the value of the coefficient .alpha. that is to be set might be forgotten.
If therefore the operation of setting the value of coefficient for realizing the two degrees of freedom for proportional control operation could be carried out smoothly, this would contribute greatly to more general application of 2 DOF PID.